Method and equipment for monitoring the current drained by the grounding electrode in electric impedance tomography

ABSTRACT

There are disclosed a method and an equipment for monitoring the current drained by the grounding electrode in an electric impedance tomography system, the said grounding electrode ( 16 ) and the set of electrodes ( 11 ) being simultaneously applied to a patient ( 12 ) in an electric impedance tomography apparatus ( 14, 15 ), comprising the conversion of the said current into a voltage signal ( 41 ) and the amplification, demodulation and filtering thereof, in order to recover its almost continuous component ( 44, 45, 46, 47, 48 ) and the comparison of the value of the latter with limit/threshold values (v 1 , v 3 , v 4 , v 6 ) proportional to the intensity of the excitation current applied to the said set of electrodes. In addition to allowing to detect the disconnection of the said grounding electrode, the said comparison allows to detect the disconnection of one or more tomography electrodes ( 11 ), unbalance in the currents injected through the said electrodes and the contact of the patient with voltage sources or conductive bodies.

FIELD OF THE INVENTION

The present invention relates to the detection of abnormal conditions of operation in electric impedance tomography systems, and refers more particularly to such conditions when associated with the grounding electrode.

DESCRIPTION OF THE PRIOR ART

Electric impedance tomography is a widely known and used technique, and consists in the positioning of a plurality of electrodes on a region of the patient, the injection of electric excitation signals between at least two of these electrodes, with simultaneous detection of the signals induced in the other electrodes and the processing of these signals in order to generate a plot indicative of the impedance in the tested region. At each measurement cycle the electrodes are set out in sequence such as to include all the electrodes installed on the patient, there being typically used systems with 32 electrodes.

These electrodes operate in differential mode, both in the cited current injection and in the operation of detection of the voltages. In order to avoid oscillations of the level of the detected signals, which would jeopardize the reconstitution of the tomography images, it is necessary to connect to the patient a floating ground electrode which function consists in draining eventual currents due to unbalance between electrodes, as well as those occurring when other equipment is used in the patient, such as an electric scalpel, and further, those caused by the contact of the patient with a conductive body (for example, the hospital bed, the side table, and other metallic utensils). That electrode should be connected away from the readout and injection electrodes in order to generate a uniform distribution of the floating ground in relation to the remaining electrodes. Such electrode is usually installed on the right or left leg region of the patient.

The disconnection of this electrode, when not detected and corrected, might generate distortions in the reconstruction of the image to be presented by the tomography equipment, and might induce errors in the interpretation of the images.

OBJECTS OF THE INVENTION

In light of what has been set forth heretofore, a first object of the invention consists in the provision of a detector capable of indicating the disconnection of the grounding electrode in an electric impedance tomography system.

One other object of the invention consists in the provision of a detector that might indicate an abnormal drainage of current caused by the use of another equipment or by the occasional contact of the patient with electrically conducting objects.

Another object of the invention consists in the provision of a detector capable of indicating the existence of an unbalanced condition in the injection of currents due to an internal imbalance of the current source of the tomography apparatus or caused by the disconnection of one or more electrodes used to inject current or to read out voltages.

BRIEF DESCRIPTION OF THE INVENTION

The cited objects, as well as others, are achieved by the present invention by means of the provision of an equipment that provides the detection of a signal corresponding to the value of the current drained through the grounding electrode as well as the treatment of this signal, subsequently conveying the same to be analyzed by means of an adequate software.

According to another characteristic of the invention, said equipment comprises means of protection against surges due to the use of a defibrillator, means for signal amplification, means for demodulation and means for analog-digital conversion of the amplified signal. Advantageously, the demodulation is carried out together with filtering that eliminates the high-frequency component of the detected signal, such component being in the range of 30 kHz to 300 kHz, as well as low-frequency oscillations above 0.5 Hz, producing as a result an almost continuous voltage.

According to another characteristic of the invention, said signal is detected in the form of a voltage measured by means of a low value resistor connected in series with the said grounding electrode.

According to still another characteristic of the invention, the proposed equipment allows the detection of eventual imbalances in the currents injected by the electrodes of the tomography apparatus, such as those caused by the disconnection of one such electrode.

According to a further characteristic of the invention, the proposed equipment allows the detection of the disconnection of the ground current drainage electrode.

According to yet another characteristic of the invention, the detection of the conditions of the drainage electrode comprises the comparison of the value of almost continuous voltage with maximum and minimum patterns.

DESCRIPTION OF THE FIGURES

The remaining characteristics and advantages of the invention will become more apparent from the description of a preferred embodiment of the invention, given for mere exemplificative purposes, and of the figures referring thereto, wherein:

FIG. 1 illustrates the arrangement of the elements in an electric impedance tomography system and the relationship thereof with the equipment of the present invention.

FIG. 2 is a block diagram illustration of the equipment proposed by the present invention.

FIG. 3 illustrates a simplified schematic diagram of the components used in the equipment according to the present invention.

FIG. 4 illustrates the screen of an oscilloscope, showing the signal detected by the resistor connected in series with the grounding electrode.

FIGS. 5 and 6 illustrate the images displayed on the screen of an oscilloscope, corresponding to the signal present at the said terminal, when the grounding electrode is connected, for two different values of the excitation current applied to the electrodes of the tomography system.

FIG. 7 illustrates the image on the screen of an oscilloscope, corresponding to the signal present at the output terminal of the circuit of FIG. 3, when the grounding electrode is disconnected.

FIG. 8 illustrates the image on the screen of an oscilloscope when displaying the signal present at the output of the circuit of FIG. 3, when the grounding electrode is connected but there is no injection of current into the electrodes of the tomography apparatus.

FIG. 9 illustrates the image displayed on the screen of an oscilloscope, corresponding to the signal present at the said terminal, when there occur anomalies in the drained currents.

DETAILED DESCRIPTION OF THE INVENTION

According to what is illustrated in FIG. 1, the arrangement used for capturing tomography images by electric impedance comprises the set of electrodes normally provided on a belt 11 around a region of the body of the patient 12, connected by means of cables 13 to the tomography apparatus 14 which generates the image displayed on the monitor 15. The grounding electrode 16 is in contact with the thigh region of the patient and is sufficiently removed from the electrodes used in the tomography to generate a substantially uniform distribution of the ground currents. This electrode is connected, by means of the shielded cable 17, to the equipment 18 that processes the signal by means of the circuit depicted in FIGS. 2 and 3, whereby the processed signal is conveyed to the digitizing and analysis device for checking of eventual anomalies. The equipment 18 and the tomography apparatus 14 are interconnected and are connected to the ground of the system 21.

According to the block diagram of FIG. 2, the object of the invention comprises the shielded cable 17 to which distal end is connected the grounding terminal 16, the proximal end of this cable being connected to the input 23 of the equipment 18, which first stage 24 consists in the means of protection against surges of voltage used in the defibrillator. Sequentially, the equipment comprises a ground draining current amplifier 25, which output is connected to a demodulator 26, provided with a low-pass filter set with a cut-off frequency setting of 0.5 Hz, which produces a substantially continuous voltage that is conveyed to the block 19, which may be integrated to the equipment that constitutes the object of the present invention or may constitute a separate unit. This block digitizes the signal by means of A/D conversion and analyzes the result, comparing the characteristics of this signal with previously established patterns, such as to detect eventual anomalies, and if required, to activate the corresponding alarms.

FIG. 3 shows, in a more detailed fashion, the schematic circuit of blocks 24, 25 and 26. For better clarity, there have been omitted in this figure the details relative to the supply of the circuit. As already mentioned, the ground drainage current captured by the electrode 16 is fed into the equipment through the shielded cable 17 which web is grounded to avoid induction arising from external field sources. The protection against high voltage values—such as those arising from the use of external equipment like, for example, an electric scalpel or a defibrillator—is provided by fast-reacting diodes 27 and 28, connected in counter-parallel fashion between the input terminals 17 a and 17 b, which divert the over-voltages to ground.

In normal operating conditions, the ground drainage current, captured by the electrode 16 and introduced into the equipment through the cable 17, flows to ground through the resistor 31. The resistance of this component should be sufficiently weak/low to avoid hindering the flow of this current. Values between 20Ω and 3000Ω are adequate for this function, there being preferably adopted a value around 100Ω. The signal resulting from the passage of this current by the said resistor has al alternate component, with the same frequency of the signal used for injecting the current into the tomography electrodes, superimposed on a direct current. This signal is introduced into the amplifier arrangement formed by the operational amplifiers 32 and 33, which amplify the said signal between 10 and 1000 times, where such amplification factor is preferably situated between 300 and 400 times. FIG. 4 represents the trace on the screen 40 of an oscilloscope showing the signal obtained at the output pin 34 of the second operational amplifier 33, the said signal presenting the alternate component 41 superimposed on a continuous voltage 43. This continuous component presents an offset ΔV with relation to the ground potential 42. The said signal is then rectified by the diode 35 and is filtered by the capacitor arrangement 36 together with the resistor 37, there resulting at the output terminal 38 an almost continuous voltage.

When the system is operating normally, this almost continuous voltage is different from zero, and this difference is proportional to the excitation current of the tomography electrodes. In the trace of FIG. 5, it is noted that in the case that this current is equal to 3 mA, the signal taken at the said output terminal 38 is presented as an almost continuous voltage 44, with a difference v2 relative to the zero axis 42, which corresponds to the ground potential in that figure. Still according to what is shown in the figure, the value of v2 is in a range comprised between the minimum v3 and the maximum v1. These values are stored in the memory of the analysis block 19, which is informed by the tomography equipment on the current that is being employed for exciting the electrodes, since this current defines the minimum limit v3 and the maximum limit v1 of the range of tolerance. Based on the cited data, the block 19 checks whether the voltage effectively measured at the terminal 38 is within the tolerance limits, driving an alarm means or equivalent resource if this does not occur.

The presence of v2 results for the imbalance of the entire system when in normal operation, when there occurs a current leak to ground. Therefore, the presence of such imbalance indicates that the tomography images obtained are reliable. On the contrary, the absence of a drained current corresponds to an abnormal operation, resulting in a jeopardized image due to a significant increase of the noise level.

The difference between the voltage captured at the terminal 38 and the zero axis is proportional to the value of the excitation current applied to the electrodes 11. In FIG. 6 there is depicted the trace on the oscilloscope screen when this current is increased to 5 mA. In this case, the observed signal 45 has a value v5 relative to the zero axis 42, and this value, compared with v2 of the previous figure appears superior thereto in a relationship approximately equal to that of the excitation currents used, that is, 5/3. The limits of the tolerance range, indicated in this figure by the minimum value v6 and the maximum value v4 shall be proportionally increased, from the information provided by the tomography apparatus 14. Also in this case, the observed trace indicates that the voltage is within the limits of the said range, and therefore the electrode 16 is correctly connected to the patient.

Should the voltage 46 shown in terminal 38 be equal to zero, as illustrated in the trace of FIG. 7, this will mean that the electrode 16 is disconnected. Such disconnection might arise from an oversight of the operator or from the movement of the body of the patient himself or herself. In both cases, the analysis performed by the block 19 will detect this abnormal condition, and if necessary, will drive an alarm device.

In FIG. 8 there is illustrated the case where the electrode 16 is duly connected to the patient, but there is not being injected current into the tomography electrodes 11. As may be observed, in such condition the signal 47 is not exactly equal to zero, and exhibits a slight offset with relation to the axis 42.

In FIG. 9 there is illustrated the trace on the oscilloscope screen when the excitation current is 5 mA, however the voltage v7 of the signal 48, measured at the terminal 38, is well above the upper limit v4 of then range of tolerance associated with this excitation current. Such condition may be due to an asymmetry in the set of drainage currents of the tomography electrodes, such as, for example, the disconnection of one or more electrodes of the set 11. One other possible cause is the connection of another equipment to the patient—such as an electric scalpel—or yet the contact of the patient with an external source of electric power.

Although the invention has been described with relation to a specific embodiment thereof, it should be understood that there may be introduced modifications thereto by technicians skilled in the art, without departing from the spirit and scope of the invention. Thus, for example, FIGS. 1 and 2 indicate the equipment that constitutes the object of the present invention as constituting a unit that is separate from the tomography apparatus, however such equipment may be incorporated to the tomography apparatus 14 without prejudice of its functionalities and characteristics.

Therefore, the invention is defined and delimited by the set of claims that follow the instant description. 

1. A method of monitoring the current drained by the grounding electrode in electric impedance tomography, said grounding electrode and a set of electrodes being simultaneously applied to a patient in a tomography apparatus by electric impedance, characterized by comprising the conversion of the said current into a voltage signal, the treatment of the said signal providing the recovery of its almost continuous component and the comparison of the value of the latter with limit values proportional to the intensity of the excitation current applied to the said set of electrodes.
 2. A method, as claimed in claim 1, characterized in that the said treatment comprises the steps of amplification, rectification and filtering of the said voltage signal.
 3. A method, as claimed in claim 2, characterized in that the said treatment further comprises the digitization of the said filtered signal prior to the comparison thereof with the said limit values.
 4. A method, as claimed in claim 1, characterized in that the disconnection of the said grounding electrode is indicated by the value substantially equal to zero of the said almost continuous component.
 5. A method, as claimed in claim 1, characterized in that the asymmetry in the drained currents of the electrodes of the tomography apparatus is indicated by a value of the said almost continuous component substantially above the upper limit of the range of tolerance of the voltage values corresponding to the current used in the excitation of the said electrodes.
 6. A method, as claimed in claim 5, characterized in that the said asymmetry is produced by the disconnection of one or more among the said electrodes.
 7. A method, as claimed in claim 1, characterized in that the contact of the patient with an external source of electrical energy is indicated by a value of the said almost continuous component substantially above the upper limit of the tolerance range of the voltage values corresponding to the current used in the excitation of the said electrodes.
 8. A method, as claimed in claim 7, characterized in that the said external source of electrical energy comprises an electric scalpel.
 9. A method, according to claim 7, characterized in that the said external source of electrical energy comprises a defibrillator.
 10. A method, according to claim 7, characterized in that the said external source of electrical energy comprises an electrically conductive body.
 11. A method, as claimed in claim 1, characterized in that the said grounding electrode is positioned substantially symmetrically with relation to the set of electrodes applied to a patient.
 12. An equipment for monitoring the current drained by the grounding electrode in electric impedance tomography, the said grounding electrode and the set of electrodes of the electric impedance tomography apparatus being simultaneously applied to the patient, characterized by comprising a low value resistor intercalated in the path of the current drained by the grounding electrode in contact with the patient and the grounding electrode of the system.
 13. An equipment, as claimed in claim 12, characterized in that the said resistor has a resistance value between 20 and 3000 ohms.
 14. An equipment, as claimed in claim 12, characterized by the provision of means of protection against voltage surges produced by the application of a defibrillator to the patient.
 15. An equipment, as claimed in claim 14, characterized in that the said protection means comprise a pair of fast-acting diodes connected in counter-parallel fashion between the input terminals of the said equipment.
 16. An equipment, as claimed in claim 12, characterized by the provision of means for amplifying the voltage signal produced by the passage of the said current through the said low value resistor, followed by means for recovery of the almost continuous component of the said signal.
 17. An equipment, as claimed in claim 16, characterized in that the said amplifying means provide a voltage gain of between 10 and 1000 times.
 18. An equipment, as claimed in claim 16, characterized in that the said amplifying means comprise one or more operational amplifiers.
 19. An equipment, as claimed in claim 16, characterized in that the said means for recovery of the almost continuous component comprise a demodulator and a filter.
 20. An equipment, as claimed in claim 19, characterized in that the said filter consists in a low-pass filter with a cut-off frequency of 0.5 Hz.
 21. An equipment, as claimed in claim 12, characterized by being incorporated into the same unit of the electric impedance tomography apparatus. 